1. Field of the Invention
The present invention relates to a multiplex communication method, a multiplex communication device and a multiplex communication system for implementing multiplex communication by means of token passing.
2. Description of the Related Art
Hitherto, there have been known remote I/O and token passing as a typical communication method of LAN (Local Area Network). FIG. 1 shows the structure of a remote I/O communication system. In FIG. 1, a plurality of terminals 2 are connected with a host unit 1 by LAN and each terminal 2 exchanges information by accessing to a host unit 1. The terminals 2 can make communication from each other only through the host unit 1 in this communication method.
When the plurality of terminals 2 access to the host unit 1 in the same time, an arbitrator within the host unit 1 arbitrates the accesses. Normally, a priority is given to each terminal 2 and the arbitrator permits the access of the terminal 2 in accordance to the priority.
FIG. 2 shows the structure of a communication system using the token passing communication method. In FIG. 2, a plurality of nodes 10 is connected in LAN and data called as token representing a right of transmission circulates on the LAN. Only node obtaining the token obtains the data transmission right. The node which has finished transmission releases the token and sends it to the next node. Thus, the communication is controlled so that the plurality of nodes does not transmit data simultaneously by causing the nodes to transmit data one after another by circulating the token.
The following problems occur when the remote I/O communication method in FIG. 1 is applied to a vehicular control system.
(1) Because only a terminal having a high priority accesses to a host unit when a plurality of terminals transmit data in the same time, terminals having low priorities have no chance to access to the host. For example, when a terminal transferring a control signal for the connection of a door has a priority lower than that of a terminal transferring an engine control signal, there is a possibility that the door does not open even when a passenger tries to escape by opening the door of the car at the time of emergency.
Meanwhile, although the problem (1) described above does not occur when the token passing communication method is adopted because each node can make own control in parallel, there arise the following problems.
(2) It has a drawback that the same time cannot be stipulated and the control cannot be made among the plurality of nodes in common because the plurality of nodes do not operate in synchronism.
It is noted that although there has been known a communication method of making a communication among a plurality of electronic control units by using a communication protocol called as CAN as a LAN communication method of a vehicular control system besides those described above, this communication method has the both problems of (1) and (2) described above.
All of the above-mentioned communication methods and other conventional communication methods have had merits and demerits to use in a control system of an apparatus such as those in a vehicle which requires reliability and there have been no suitable communication method.